Refractometer for liquids



Jan. 3, 1956 B. B. BRIANT 2,729,135

REFRACTOMETER FOR LIQUIDS Filed Dec. 18, 1952 v 2 Sheets-Sheet 1 v 605 5ff/4N7 INVENTOR. Ere. 2

Jan. 3, 1956 I BRIANT 2,729,135

REFRACTOMETER FOR LIQUIDS E125 ELI-L5 E11? ArroPA/f Y United StatesPatent 2,729,135 REFRACTOMETER FOR LIQUIDS Bob -B. Briant, Wichita,Kans., assignor to Boeing Airplane Company, Wichita, -KEIHSincorporation of-Deiaware Application December 18, 1952, Serial No.326,743 2Claims. (Cl. 88- 1-4) This invention relates to an improvedrefractometer, and more particularly to an improved instrument of thistype for accurately determining the indices of refraction of variousliquids.

Instruments for this purpose are presently available, but most of themare ,both expensive and complicated. An interferometer, for example,requires the visual matching of interference fringes .by a highlyskilled operator. Such procedure is tiring, and accuracy dependson theoperator, thus makingsuch instruments subjective in their method ofoperation. The same applies to reflectometers.

This invention provides a much simpler and more rugged instrument whichis accurate, yet which is objective in its method of operation,and.therefore does not require a skilled operator.

This invention is basedon the theory that the 'focal length of acondensing lens is a function of the radii of curvature of the lensfaces and the index of refraction of the substance. of which the lens ismade. If the lens substance is a liquid, then its index of refractioncan be obtained by first accurately determining its focalllength, andcomparing that focal length with the focal length of a similar lens of aliquid whose index of refraction is known. Rather than actuallyrequiring a comparison, however, the instrument is accuratelycalibrated, and the index of refractionof the liquid being tested isread directly. i

It is the chief object of the invention to provide a rugged instrumentwhich is simple in construction, and which may be used by unskilledoperators at points remote from any laboratory; one which can betransported from place to place without destroying its accuracy.

It is a further object to provide an accurate refractometer which,omitsmany expensive and complicated components which are included inmost presently available refractometers. Forinstance, a refractometerembodying my invention includes no observation telescope, no filters for.producing ,a monochromatic light beam,.no collimating lens, no seriesof prisms, no mirror reflector,-no photoelectric cellor other splitradiation detector, no differential glavanometer, ,and no differentialelectronic amplifier.

The invention, together with other objects, will'be more clearlyundersoodwhen the following description is read in connection with thedrawings, in which:

Fig. 1 is a perspectiveviewof a complete refractorneter embodying theinvention, parts being cut away for clarity; i

Fig. 2 is a sectional view showing details in construction of certainparts;

.Fig. 3 is a plan view of a clamp, portions being shown in section todisclose details in construction;

Fig. 4 is a schematic view illustrating the principle of operation ofthe refractometer;

' Fig. 5 is a plan view of a preferred form of light shield forming apart of the invention; and

Figs. 6 and 7 are front views through an observation being tested.

General construction and operation Fig. 4 illustrates schematically thegeneral construction and operation of a refractometer embodying myinvention. A condensing lens 15 is a liquid filled substantiallycylindrical thin walled glass tube or bottle. An image producing shieldor blanking screen 16 is fixed adjacent the bottle wall, and is providedwith spaced apertures 17 and 18, spanned by image producing strands. Asource of light 19 is spaced from shield 16, and light rays from thissource pass through apertures 17 and 18, through the lens 15, aredeflected inward thereby, and intersect at some point spaced from theopposite side or the lens. A thin translucent image receiving screen 20is mounted parallel to shield 16 for movement toward and away from thelens 15 along a line passing through the axes of the lens and of thelight source. The screen 20 serves as a detector to determine the exactpoint at which the lens deflected light rays intersect. The convergingrays are clearly visible on the surface of the screen, and when it ismoved to the proper position the .two light images coincide and appearas a single image. Since the distance between the lens and the point ofintersection of the deflected rays is the focal length of the lensformed by the liquid and the bottle, .it is only necessary to provide amoderately fine micrometer screw (not shown in Fig. 4) to move screen.20 toward and away from the lens, and to properly calibrate the screw,to convert various focal lengths directly into index of refractionreadings.

Detailed construction Referring to Fig; 1, the refractometer illustratedincludes a box-like housing 21 having an open observation aperture 22 inone of its ends. Interiorly the side Walls of the housing carry inwardlyprojecting oppositely positioned and' parallel ribs 23-24 and 2526,which together form two vertically spaced parallel slideways.

A lens holding clamp 27 is mounted for slidable movement on the twoupper ribs 23 and 24. This clamp is made up of two halves 28 and 29,each having slots in its opposite ends to receive and cooperate withthe, ribs. Along their adjacent side edges the two halves are recessed,as shown in Fig. 3, to form a lens receiving socket between them.Clamping pressure is provided by va pair of tension coil springs 30 and31, the opposite ends of which are enclosed in aligned recesses in therespective halves, and anchored by pins 32 and ,33. The springs normallymaintain the adjacent side edges of the .two halves in contact. In use,one half 28 is secured against movement along theribs by means of setscrews 34and 35. The block 29 may thus be grasped manually and pulledslightly away from block 28 to open the socketso that the lens may beinserted. When block 29 is released, spring pressure clamps the lens 15between the blocks at any desired elevation. It will be understood, ofcourse, that other types of clamps will serve as suitable substi tutesfor the one described.

On the lower set of ribs 25 and 26, a traveling focal length detector,designated as a whole by the numeral 36, is slidably mounted by means ofrib receiving slots 37 in each of its side edges. Detector 36 has athreaded longitudinally extending bore which receives the exteriorlythreaded sleeve 38 of a compound micrometer screw, which will presentlybe described in detail. At one end, the detector 36 carries an uprightplate 39 which is pro vided with a central viewing aperture 40 which iscentrally .aligned with aperture 22 in the housingiend wall.

The plate 39 is slotted transversely to receive and resiliently grip areversely folded sheet 41 of thin gage metal which holds in its fold avery thin sheet of translucent material, such as frosted glass,Plexiglas, silk, or oiled paper, which constitutes an image receivingscreen 42. The two sides of folded sheet 41 are provided with concentricapertures 43 and 44 (Fig. 2), which are also concentric with plateaperture 40 and housing wall aperture 22. A line passing through thecenter of the source of light and through the longitudinal axis of thelens also passes through the center of all the mentioned apertures 40,44, 43 and 22. With the entire detector 36 mounted on threaded sleeve 38as a traveling nut, it can readily be seen that rotation of the sleevewill move translucent focal length detector screen 42 toward and awayfrom lens 15.

The previously mentioned compound micrometer screw includes the sleeve38 and a fixed screw 45, one end of which is received in acomplementally threaded bore 46 of the sleeve, and the other end ofwhich threadedly engages and projects through the back wall 47 of thehousing. Rotation of this screw is normally prevented by a lock nut 48,which may be loosened and the position of the screw adjusted by means ofa screw driver slot 49 in the end of the screw.

By externally threading sleeve 38 with right hand threads, 28 per inch,and screw 45 with right hand threads, per inch, an extremely finemicrometer screw is produced. Actually sleeve 38 may be rotated 28complete revolutions while moving screen 42 only four-tenths of one inchtoward or away from lens 15. A pair of laterally spaced tension coilsprings 50 each has one end secured to the detector 36 and its other endsecured to the front wall of the housing. These springs 50 serve toeliminate any possible lead error in the threads on the detector, thesleeve 38, or the screw 45 Sleeve 38 is unthreaded near its outer end,and is journaled in the front wall of the housing. That portion whichnormally projects outward from the housing Wall is accurately calibratedfor direct reading of indices of refraction. The outer surface of thewall immediately surrounding the sleeve is calibrated in tenths toprovide a Vernier scale. A knurled knob 51 is mounted on the extremeouter end of sleeve 38 to facilitate its adjustment.

A small electric bulb 52 is mounted in a bracket support 53 on the innersurface of the back wall 47, as shown, and serves as the source of lightpreviously mentioned.

The twin apertured shield 16 is secured, as by screws, to the nethersurface of clamp half 29, as clearly shown in Fig. 1. Referring to Fig.5, it will be seen that fine parallel vertically disposed hairs or webstrands 54 and 55 are secured centrally across the shield apertures 17and 18. Since it is desirable for the sake of accuracy to projectextremely fine images on translucent detector screen 42, short lengthsof spider web strands are preferably used instead of hairs. The strands54 and 55 are preferably cemented directly to the surface of shield 16.

Operation A selected liquid is placed in the lens bottle, and the bottleis positioned in clamp 27. When bulb 52 is energized light rays passthrough shield apertures 17 and 18, through the condensing lens formedby the bottle walls and the liquid, and hairline images are projectedonto the surface of translucent detector screen 42, as clearly shown inFigs. 6 and 7. In Fig. 6, it is evident that screen 42 has not yet beenmoved to the exact plane in which the lens deflected rays intersect. Bysimply turning knob 51 in one direction or the other, detector 36 movesalong sleeve 38 to move screen 42 into the proper plane so that the twoseparate projected images exactly coincide, and appear as a singleimage, as in Fig. 7. Thus, a null reading is obtained.

By then reading the calibrations on the outer end of sleeve 38, theindex of refraction of the liquid in the bottle is obtained. In Fig. 1,the reading is 1.2423.

While it is true that the radius of the lens remains constant, and thatthe lens substance (the liquid) is a variable, the distance the lightrays travel through the bottle walls is very small compared to thedistance which the rays travel through the liquid. Any deviation due tothe glass portion of the lens is eliminated during calibration of thesleeve 38, and therefore has no effect on the final readings. Effects oftemperature variations on the lead screws 38 and can easily becompensated for by filling the bottle with a liquid whose index ofrefraction is known, by adjusting knob 51 until the scale reads theknown index of refraction, by loosening nut 48 and adjusting screw 45until the two light projected images of strands 54 and coincide onscreen 42, as in Fig. 7.

From the above description, it is evident that this invention provides ameans of accurately determining the focal length of a condensing lenswhich is made up primarily of the liquid being tested, and convertingthat focal length into an index of refraction reading for the particularliquid of which the lens is formed. While only a cylindrical condensinglens is shown in the drawings, it will be understood by those familiarwith this art that other types and shapes of liquid filled condensinglenses will serve equally well.

Having described the invention with sufficient clarity to enable thosefamiliar with this art to construct and use it, I claim:

1. A device for determining the index of refraction of a substancecomprising, in combination: a thin walled cylindrical transparentcontainer for holding the substance, the substance and the containerwall together constituting a condensing lens; a source of light on oneside of said lens; an apertured otherwise light impervious imagecreating shield interposed transversely between the lens and the lightsource; a translucent image receiving focal length detecting screen onthe opposite side of the lens, the light source, the shield, the lensand the screen all being in optical alignment, said shield having twoapertures spaced equidistant from the line of optical alignment onopposite sides thereof; means supporting said container in fixedposition with relation to said shield and the source of light; a movablemember supporting said screen in a plane substantially perpendicular tothe line of optical alignment; a compound micrometer type screwsupporting said movable member for movement along the line of opticalalignment; means journaling the screw; and cooperating means on thescrew and on the screw journaling means to measure a distance betweenthe lens and screen proportional to the index of refraction of the lenssubstance when the two light images created by the shield coincide onthe screen.

2. The device described in claim 1, and means for adjusting one part ofthe compound screw with relation to another part to compensate forlength variation in the screw due to temperature changes.

References Cited in the file of this patent UNITED STATES PATENTS236,911 Scharpf Jan. 25, 1881 1,988,169 Duckwall Jan. 15, 1935 2,595,495Von Berg et al. May 6, 1952 FOREIGN PATENTS 5,281 Great Britain of 189211,200 Great Britain of 1905 15,535 Great Britain of 1905 313,261 GreatBritain June 13, 1929 591,991 Great Britain Sept. 4, 1947

1. A DEVICE FOR DETERMINING THE INDEX OF REFRACTION OF A SUBSTANCECOMPRISING, IN COMBINATION: A THIN WALLED CYLINDRICAL TRANSPARENTCONTAINER FOR HOLDING THE SUBSTANCE, THE SUBSTANCE AND THE CONTAINERWALL TOGETHER CONSTITUTING A CONDENSING LENS; A SOURCE OF LIGHT ON ONESIDE OF SAID LENS; AN APERTURED OTHERWISE LIGHT IMPERVIOUS IMAGECREATING SHIELD INTERPOSED TRANSVERSELY BETWEEN THE LENS AND THE LIGHTSOURCE; A TRANSLUCENT IMAGE RECEIVING FOCAL LENGTH DETECTING SCREEN ONTHE OPPOSITE SIDE OF THE LENS, THE LIGHT SOURCE, THE SHIELD, THE LENSAND THE SCREEN ALL BEING IN OPTICAL ALIGNMENT, SAID SHIELD HAVING TWOAPERTURES SPACED EQUIDISTANT FROM THE LINE OF OPTICAL ALIGNMENT ONOPPOSITE SIDES THEREOF; MEANS SUPPORTING SAID CONTAINER IN FIXEDPOSITION WITH RELATION TO SAID SHIELD AND THE SOURCE OF LIGHT; A MOVABLEMEMBER SUPPORTING SAID SCREEN IN A PLANE SUBSTANTIALLY PERPENDICULAR TOTHE LINE OF OPTICAL ALIGNMENT; A COMPOUND MICROMETER TYPE SCREWSUPPORTING SAID MOVABLE MEMBER FOR MOVEMENT ALONG THE LINE OF OPTICALALIGNMENT; MEANS JOURNALING THE SCREW; AND COOPERATING MEANS ON THESCREW AND ON THE SCREW JOURNALING MEANS TO MEASURE A DISTANCE BETWEENTHE LENS AND SCREEN PROPORTIONAL TO THE INDEX OF REFRACTION OF THE LENSSUBSTANCE WHEN THE TWO LIGHT IMAGES CREATED BY THE SHIELD COINCIDE ONTHE SCREEN.